Ohm alone, on the phones.

For a given volume control setting, headphones having a lower impedance will draw more milliwatts (mw) than your 64 ohm phones. That may or may not enable you to reduce the setting of the volume control, depending on how the efficiencies of the two phones compare. And if the efficiency of the lower impedance phones is significantly less than the efficiency of your 64 ohm phones you would have to **increase** the setting of the volume control.

Headphone efficiency is usually specified as the sound pressure level (SPL) produced in response to an input of 1 mw. Sensitivity is sometimes specified instead, in the form of the SPL produced in response to an input of 1 volt. If necessary, an apples-to-apples comparison of the two kinds of specs can be made by converting one into the other based on the formula:

P = (E squared)/R

Where P is power (in watts), E is voltage (in volts), and R is impedance (in ohms).

All of that being said, Kijanki makes a good point that reducing the setting of the volume control (by using higher efficiency phones) may not help anyway, depending on where the noise is being generated and on where in the signal path the volume control is located. And since it appears that the noise is being generated "upstream" of the volume control in this case, his comment about volume controls and signal-to-noise ratios seems particularly likely to be applicable.

Regards,
-- Al

Curses!  Foiled again!

Thanks.

It can only make it worse IMHO.  If the hiss comes from the output stages then lower ohm ratings will make it worse.  Hiss coming from the input stages will appear the same, since volume control won't change signal to noise ratio.